The present invention relates to the purification of 1,2-dichloroethane (ethylene dichloride), and more particularly to an improvement of a method for purifying crude 1,2-dichloroethane by employing a distillation column of plate type or packed type.
At the present day, vinyl chloride has been in general prepared industrially by a process in which purified 1,2-dichloroethane is thermally cracked. Crude 1,2-dichloroethane produced by oxychlorination of ethylene (hereinafter referred to as "oxy-EDC"), crude 1,2-dichloroethane produced by direct chlorination of ethylene (hereinafter referred to as "direct-EDC") and uncracked 1,2-dichloroethane (hereinafter referred to as "uncracked-EDC") recovered in the thermal cracking of 1,2-dichloroethane are purified and employed as a feed 1,2-dichloroethane in the production of vinyl chloride monomer (hereinafter referred to as "VCM") by an oxychlorination process.
The crude oxy-EDC usually contains as impurities low-boiling compounds (having a boiling point of not more than 83.7.degree. C./760 mmHg) such as ethyl chloride, cis-dichloroethylene, chloral, carbon tetrachloride and trichloroethylene and high-boiling compounds (having a boiling point of not less than 83.7.degree. C./760 mmHg) such as 1,1,2-trichloroethane and tetrachloroethane. On the other hand, the crude direct-EDC usually contains as impurities low-boiling compounds such as ethyl chloride, 1,1-dichloroethane and chloroform and high-boiling compounds such as 1,1,2-trichloroethane and tetrachloroethane. Also, the uncracked-EDC usually contains as impurities low-boiling compounds such as ethyl chloride, chloroprene and chloroform and high-boiling compounds such as monochlorobenzene.
A distillation column has been employed for purifying these crude 1,2-dichloroethanes of three kinds to the extent such that 1,2-dichloroethane (hereinafter referred to as "EDC") can be cracked without any problems to produce VDC of usual quality. FIG. 1 is a flow sheet showing a conventional purification method using a distillation column. In general, the purification is carried out by passing the crude EDC first through a so-called low boiler column I for removing impurities having lower boiling points than EDC and then through a high boiler column II for removing impurities having higher boiling points than EDC. In a conventional purification method, in order to remove the low-boiling compounds, the above-mentioned crude EDC of three kinds are usually mixed at an appropriate place in an appropriate manner and are fed through a feed pipe 1 to a certain appropriate one plate of the low boiler column I, and low-boiling compounds are distilled from the top of the column through a pipe 3. The bottom product is then fed through a pipe 4 to the high boiler column II, and the purified EDC is recovered from the top of the column through a pipe 5, while high-boiling impurities are removed from the bottom of the column through a pipe 6. In the low boiler column I, it is necessary to purify the crude EDC so that EDC recovered as a bottom product from the bottom of the column contains no low-boiling compounds, or even if contians, the amount of low-boiling compounds is so small that they do not hinder the thermal cracking of EDC in the production of vinyl chloride, in other words, do not decrease the thermal cracking rate and also do not cause clogging of a reaction tube, e.g. less than several hundreds p.p.m. When carbon tetrachloride included in the crude oxy-EDC, which is effective as a catalyst for thermal cracking of EDC, is recovered as a bottom product together with EDC, it is possible to conduct the thermal cracking of EDC at a relatively low temperature, since the purified EDC contains the carbon tetrachloride catalyst.
However, in a conventional distillation method as mentioned above in which the mixed crude EDC is fed to a certain one plate of a distillation column, the attempt of including carbon tetrachloride into the purified EDC without increasing the feed amount of steam has accompanied the defect that the concentration of other low-boiling compounds in the purified EDC also increases. On the other hand, when it is attempted to decrease the concentration of low-boiling compounds, carbon tetrachloride is also distilled away together with other low-boiling compounds. Further, when the amount of supplied steam is increased in order to raise the concentration of carbon tetrachloride in the purified EDC with keeping the concentration of other low-boiling compounds low, not only the utility cost increases, but also the concentration of carbon tetrachloride largely changes by a slight change in the conditions of operating a low boiler column, so upon preparing VCM by thermal cracking of the thus purified EDC, the conversion is not stabilized.
Accordingly, it is an object of the present invention to provide an improved method for purifying the crude oxy-EDC, crude direct-EDC and uncracked-EDC by employing a distillation column, which has a great effect in steam saving.
A further object of the invention is to provide an improved method of purifying crude EDC by which, in one distillation column for removing low-boiling impurities, the fluctuation range of the intended concentration of carbon tetrachloride contained in a bottom product can be controlled within the range of .+-.500 p.p.m. over a wide range of the concentration of carbon tetrachloride, practically 0 to 8,000 p.p.m., so that the conversion of the purified EDC into vinyl chloride by thermal cracking is stabilized.
Another object of the present invention is to provide a method for producing VCM having a stable quality with a decreased amount of energy from a crude EDC by subjecting the purified EDC, obtained by the above purification method, having a stabilized concentration of carbon tetrachloride, to thermal cracking at a high and stable rate of thermal cracking.
These and other objects of the present invention will become apparent from the description hereinafter.